1. Field of the Invention
The present invention relates to color measuring apparatus and method capable of suitably measuring color, for example, on a display surface of a liquid crystal display device.
The present invention also relates to a liquid crystal display system capable of color calibration on a display surface of a liquid crystal display device based on the color measured by the color measuring apparatus.
2. Description of the Background Art
Conventionally, CRT displays have been a mainstream of display devices, but devices adopting various methods such as liquid crystal displays (LCDs), plasma displays (PDPs) and organic EL displays have been developed in recent years and are spreading. Such a display device is required to be of a relatively high quality due to its intended use, for example, in applications such as printing application and medical application and a calibration process for calibrating luminance and chromaticity deviations is performed to satisfy this requirement.
Upon this calibration, luminance and chromaticity need to be measured and a luminance meter is used to measure luminance. Known luminance meters include telescopic luminance meters and contact luminance meters. A telescopic luminance meter is an apparatus for measuring the luminance of a display device from a position at a specified distance from the display device and includes an optical system inside. Since light in a limited range is condensed by this optical system, a measurement field is narrow. On the other hand, a contact luminance meter is an apparatus for performing a measurement while being held in contact with a display surface of a display device, and is so structured as to be usable while being attached to the display surface of the display device by means of a sucking disk or the like. A distance from the display surface of the display device to a light receiving sensor is short, no such optical system as to narrow a light receiving angle is provided in many cases for cost reduction and a measurement field is relatively wide.
A color sensor called a CRT calibrator produced by Konica Minolta Holdings, Inc is, for example, known as such a contact luminance meter. This CRT calibrator is roughly such that incident light is received by silicon photodiodes arranged in a one-to-one correspondence with color filters of X, Y and Z in the so-called CIE color systems via the respective color filters, light receiving outputs of the respective silicon photodiodes are converted by a current-to-voltage conversion circuit and an analog-to-digital conversion circuit, digital values corresponding to the respective silicon photodiodes are taken into a built-in microcomputer, a colorimetric value as a final output value is calculated based on these respective digital values while calibration is performed by this microcomputer and the colorimetric value is output.
In the case of measuring luminance on a display surface of a CRT display by such a luminance meter, substantially the same value is obtained regardless of whether the measurement is conducted by a telescopic luminance meter or by a contact luminance meter and an accurate measurement is possible since a luminance variation according to a viewing angle is relatively small and the viewing angle dependence of the luminance is small in the CRT display. However, the luminance largely varies according to the viewing angle and the viewing angle dependence of the luminance is large at gradations in a dark part of the liquid crystal display, wherefore no accurate measurement can be conducted by the contact luminance meter.
In order to accurately measure luminance on a display surface of a liquid crystal display using this contact luminance meter, there is a luminance measuring apparatus for liquid crystal display, for example, disclosed in Japanese Unexamined Patent Publication No. 2003-294528 (U.S. Pat. No. 6,657,712, D1). The luminance measuring apparatus for liquid crystal display disclosed in this document D1 is provided with a contact luminance meter, a light blocking cushion member surrounding a light receiver of the contact luminance meter, a light meter including a fixture for moderately pressing the light blocking cushion member against a display surface of the display and fixing the contact luminance meter to the liquid crystal display so that a facing direction of the light receiver of the contact luminance meter and that of the display have a fixed relationship, a converter for converting a luminance measurement result by the contact luminance meter into the one corresponding to a telescopic luminance meter based on the luminance measurement result by the contact luminance meter and that by the telescopic luminance meter, and a processor for converting the luminance measurement result by the light meter using the converter. The document D1 discloses that, by the above construction, the display luminance of the liquid crystal display can be accurately measured by the contact luminance meter as if it were measured by the telescopic luminance meter.
In the case of measuring the liquid crystal display, the above apparatus can measure bright gradations with relatively high accuracy, but cannot properly correct in the measurement at a dark gradation and it is difficult to measure with high accuracy since the viewing angle dependence of the liquid crystal display is large.